Injector apparatus having a clutch to inhibit forward movement of the plunger

ABSTRACT

An injector apparatus includes a housing ( 10   1   , 10   2 ) for a cartridge or syringe, a plunger ( 40 ) for cooperating in use with the cartridge or syringe to express successive doses, a dose setting arrangement ( 16, 20 ) to select a dose volume, and a drive mechanism ( 22, 24 ) releasable to advance the plunger in respective predetermined increments of magnitude to express the successive doses. The drive mechanism includes a drive spring ( 22 ) for imparting movement directly or indirectly to the plunger ( 40 ), and the dose setting arrangement includes a dose setting element ( 16 ) moveable in a dose setting routine to define a magnitude of an increment of movement of the plunger for a given dose. The drive mechanism further includes a clutch arrangement ( 56 ) operable during the dose setting routine to inhibit forward movement of the plunger and/or to isolate the dose setting element from the force of the drive spring.

This invention relates to injector apparatus and in particular, but notexclusively, to such apparatus used for injecting multiple doses ofvariable magnitude, for example of insulin.

BACKGROUND OF THE INVENTION

In our earlier Autopen® device, a rotatable dose setting knob attachedto the rear end of the pen injector is connected to a hollow drivesleeve that carries an annular ratchet surface at its front end whichengages a corresponding ratchet surface on a threaded drive collar. Thethreaded drive collar is threadedly engaged with the stem of a plungerso that rotary movement of the drive collar is converted into a linearadvance of the plunger. A trigger can be moved to release the drivecollar for rotary movement. During a dose setting routine, with thedrive collar held against rotation by the trigger, a dose is dialled inby rotating the dose setting knob and the drive sleeve against a springbias provided by a main drive spring. The dose setting movement isunidirectional only due to the ratchet action between the drive sleeveand the drive collar. Upon release of the trigger, the drive collarrotates by an angular amount equivalent to the angular amount initiallydialled in, and the plunger is advanced by the corresponding amount todeliver the dose of the required number of units.

This device performs extremely well and enjoys considerable success butwe have identified a number of improvement opportunities. In the abovedevice, in setting a dose the user winds up the spring to provide themotive force that is needed for the next dose. This means that it can beawkward for those of limited dexterity to exert sufficient grip torotate the dose setting knob against the bias of the drive spring. Alsoin this design, because the action of dose setting winds up the drivespring, a ratchet action is required so that when the user releasestheir grip on the dose setting knob, the knob stays in that position anddoes not immediately return to the zero position under the influence ofthe drive spring. To allow dose setting reversal in case of settingovershoot, a bi-directional ratchet mechanism is then needed, orotherwise a disconnection mechanism is required, either of whichcomplicate the device.

SUMMARY OF THE INVENTION

We have therefore designed an injector apparatus in which a clutcharrangement is provided to isolate the dose setting element from theforce of the drive spring during the dose setting routine.

Accordingly, in one aspect, this invention provides an injectorapparatus for use with a cartridge or syringe to deliver a plurality ofdoses therefrom, the injector apparatus comprising:

-   -   a housing for the cartridge or syringe;    -   a plunger for cooperating in use with the cartridge or syringe        to express successive doses;    -   a dose setting arrangement to select a dose volume;    -   a drive mechanism releasable to advance said plunger in        respective predetermined increments of magnitude to express said        successive selected doses;    -   characterised in that said drive mechanism includes a drive        spring for applying expulsion drive movement directly or        indirectly to said plunger, and said dose setting arrangement        comprises a dose setting element moveable in a dose setting        routine to set a magnitude of an increment of movement of said        plunger for a given dose, said drive mechanism further including        a clutch arrangement operable during said dose setting routine        to isolate said dose setting element from the force of said        drive spring and optionally to inhibit directly or indirectly        forward movement of the plunger.

In an arrangement where the dose setting element is isolated from theforce of the drive spring during the dose setting routine, the dosesetting element can be rotated easily, and also preferably can be movedin the reverse sense if the user moves the dose setting element past arequired dose. Isolating the dose setting element means that thedose-setting movement of said dose setting element is not resisted bysaid spring.

It follows from the above that if the dose setting action does notenergise the drive spring, the drive spring needs to be energised insome fashion. Conveniently, the drive spring is preloaded to an extentsufficient to deliver substantially the entire useable contents of thesyringe or cartridge in a succession of doses. Where the dose settingelement is isolated from the force of the drive spring during the dosesetting routine, a more powerful spring may be used than would beappropriate for the conventional user-wound device. Instead of providinga fully preloaded drive spring, the drive spring could be partiallypreloaded or with no preload but with suitable separate means forenergising the spring. It will also be appreciated that in sucharrangements the drive mechanism will usually already be under load fromthe drive spring prior to dose setting, in contrast to prior art deviceswhere the drive mechanism is under load only once the dose has beendialled in.

Preferably, the dose setting element is moveable angularly to set saidincrement of plunger movement.

The clutch arrangement preferably includes a clutch element moveablebetween disengaged and engaged positions. Thus the clutch element may beadapted to move axially with said dose setting element, whereby axialmovement of said dose setting element from a first, rest, position to asecond, dose setting position causes said clutch element to engage toinhibit movement of said plunger. It is also convenient for the dosesetting element and the housing to include complementary abutments thatengage to prevent relative rotation of said dose setting elementrelative to said housing when the dose setting element is in its restposition, but which disengage to allow dose setting rotation of the dosesetting element when it is in its setting position. In this manner, thedose setting element may be moved axially from its rest position to itsdose setting position, then turned to dial in a required dosage volume,and then returned axially to its rest position to lock it againstrotation.

Preferably, upon moving the dose setting element from its rest positionto its setting position, the clutch engages to inhibit movement of thedrive plunger a predetermined distance before the abutments on the dosesetting element and the housing disengage to release the dose settingelement for dose setting movement.

In a preferred arrangement, when the dose setting element is in its restposition, it receives and reacts the thrust of the drive spring so thatthe dose setting element thereby inhibits movement of the drive plunger.In this manner, when the dose setting element is in its rest positionthe thrust of the drive spring is reacted by the dose setting elementand thence to the housing, but when the dose setting element is in itssetting position, the thrust of the drive spring is reacted by theclutch member having engaged with the housing.

Preferably, having set a dose, returning the dose setting elementaxially to its first position disengages the clutch element to releasethe drive mechanism to cause the plunger to advance by a predeterminedincrement corresponding to the predetermined set dose.

Preferably, said plunger is threadedly engaged with an associated driveor control element whereby advance of said plunger is accompanied byrotation of the plunger or the drive or control element, with themagnitude of the incremental advance being set by constraining saidrotation.

Preferably a stop member is associated with the dose setting element andis adjustable by moving the dose setting element, thereby to define anangular increment for the relative rotation between the plunger and thedrive or control element upon release of the drive mechanism.

The stop member could simply be an abutment surface provided on the dosesetting element. This would allow the maximum useable extent of angularmovement of the dose setting element to be just short of 360°. Thiswould mean that the indicia needed to be fairly closely packed in someinstances. Accordingly it is preferred for there to be an intermediatedial or shuttle member threadedly engaged with the dose setting elementand constrained to rotate with the rotatable one of the plunger and thecontrol member, with the extent of relative angular movement of the dialor shuttle member and the dose setting element being set by the relativeposition of the stop member associated with the dose setting element.The provision of a threaded dial or shuttle member means that it is nowpossible to set a dose of several turns. This has advantages both interms of the available size for the indicia, and also allows greaterflexibility over the choice of the pitch of the thread between theplunger and the control member. In a particularly preferred arrangement,the dosage indicia may be provided as a helical strip on one of saidcomplementary dose setting elements, and read off via a marker or windowon the other dose setting element.

The drive spring in the various arrangements described above may beeither a torsion spring that imparts a rotary movement when released, ora compression spring that imparts a linear movement.

In many applications it may be desirable to provide a multiple doseinjector device that is supplied preloaded so that it is not necessaryto re-energise the drive between each dose, for example if the device isintended to be disposable.

Accordingly, in another aspect, this invention provides an injectorapparatus for use with a cartridge or syringe to deliver a plurality ofdoses therefrom, the injector apparatus comprising:

-   -   a housing for the cartridge or syringe;    -   a plunger for cooperating in use with the cartridge or syringe        to express successive doses;    -   a dose setting arrangement to select a dose volume;    -   a drive mechanism releasable to advance said plunger in        respective predetermined increments of magnitude to express said        successive doses;

characterised in that said drive mechanism includes a rotary storedenergy element with a preload sufficient to deliver at least a pluralityof said successive doses.

In the above arrangement, the injector apparatus may for example beprovided with a fully preloaded spring so that the user is not requiredto input the mechanical energy required to express the doses. Thisrenders the device particularly suitable for those with limiteddexterity or poor grip.

As noted above, conventional Autopen® and similar devices have anangular range of dose setting movement that is limited to 360°, and thisplaces constraints on the marking indicia, the pitch of the drivethread, and the properties of the drive spring. We have thereforedesigned an arrangement in which the dose setting arrangement includesthreadedly engaged first and second complementary dose setting elementswhich enables the dose setting movement to be several turns if required.

Accordingly, in another aspect of this invention, there is provided aninjector apparatus for use with a cartridge or syringe to deliver aplurality of doses therefrom, the injector apparatus comprising:

-   -   a housing for the cartridge or syringe;    -   a plunger for cooperating in use with the cartridge or syringe        to express successive doses;    -   a dose setting arrangement to select a dose volume;    -   a drive mechanism releasable to advance said plunger in        respective predetermined increments of magnitude to express said        successive doses and in which advance of said plunger is        controlled by a control member that moves angularly in response        to advance of said plunger;

characterised in that said dose setting arrangement includes first andsecond complementary dose setting elements threadedly engaged forrelative threaded movement away from a limit position, with said firstdose setting element being moveable relative to said housing in asetting routine from a rest position to a variable angular position thatsets the dose volume, and said second dose setting element beingconstrained to rotate with said control member,

whereby on release of said drive mechanism said control member and saidsecond dose-setting element rotate until the second element returns tosaid first limit position with respect to the first element, therebypreventing further rotation of the control member.

According to another aspect, the invention provides an injectorapparatus for use with a cartridge or syringe to deliver a plurality ofdoses therefrom, the injector apparatus comprising:

-   -   a housing for the cartridge or syringe;    -   a plunger for cooperating with the cartridge or syringe to        express successive doses;    -   a drive mechanism energised by a drive spring, and releasable to        advance said plunger in predetermined increments as determined        by adjustment of a dose setting element, and    -   a re-energising element for re-energising said spring        independently of movement of said dose setting element.

Preferably said drive spring is a torsion spring and said re-energisingelement is a rotary element, for example a manually rotatable element.

According to another aspect, this invention provides an injectionapparatus for use with a cartridge or syringe to deliver a plurality ofdoses therefrom, the injector apparatus comprising:

-   -   a housing for the cartridge or syringe;    -   a plunger for cooperating in use with the cartridge or syringe        to express successive doses;    -   a dose setting element movable to select a dose volume;    -   a drive mechanism releasable to advance said plunger in        respective predetermined increments of magnitude to express said        selected dose volumes, said drive mechanism including a drive        spring for providing motive force directly or indirectly to said        plunger to advance said plunger, and    -   the apparatus being configurable between a first position in        which the motive force of the drive spring is transmitted via        the dose setting element to the housing to be reacted thereby,        and a second position in which a clutch element is moved into a        position in which motive force of the drive spring is        transmitted via the clutch element to the housing to be reacted        thereby, so that dose setting movement of the dose setting        element is not resisted by the drive spring.

Whilst the invention has been described above, it extends to anyinventive combination of the features set out above or in the followingdescription of claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be performed in various ways, and an embodimentthereof will now be described by way of example only, reference beingmade to the accompanying drawings in which:

FIG. 1 is a side view of a first embodiment of pen injector inaccordance with this invention:

FIG. 2 is a side section view of the pen injector of FIG. 1;

FIG. 3 is an exploded view of the pen injector of FIGS. 1 and 2;

FIG. 4 is an enlarged view of the main body of the pen injector of FIGS.1 to 3, containing the drive mechanism, with one body half showntransparent;

FIG. 5 is a perspective section view of the dose setting knob showingthe thread on the inside of the dose setting knob;

FIG. 6 is a perspective section view on the rear part of the drivemechanism showing the clutch and dose setting knob and dose dial;

FIG. 7 is an exploded view of a second embodiment of pen injector inaccordance with the invention having a rewind facility;

FIG. 8 is a detailed view of the rewind knob, and

FIG. 9 is a side section view of a third embodiment of pen injectordriven by a compression spring.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiment of pen injector illustrated in FIGS. 1 to 6 is designedto be a disposable automatic pen type injector capable of expelling asuccession of doses of variable preset magnitude from a cartridge. Theinjector comprises a body made up of symmetric body halves 10 ¹ and 10 ²which may be snap-fitted or glued together. A cartridge or syringe 12 isreceived in a transparent forward cartridge housing 14 which is asnap-fit on the forward end of the body halves 10 ¹, 10 ². On the rearend of the body is mounted a dose setting knob 16 having a window 18through which a dose dial 20 is visible. As to be described below, thedevice contains a preloaded torsion spring which in this particularembodiment supplies the entire force required to express all the useabledosage volume in the cartridge. Doses are set by pulling the dosesetting knob 16 rearwardly, rotating it until the required dosage unitis visible on the dose dial and then pushing the dose dial back in torelease the drive mechanism to expel the required dose.

Referring now more particularly to FIGS. 2, 3 and 4 the body 10 ¹, 10 ²defines an internal cylindrical space in which a torsion spring 22 islocated in an energised state, with the forward end of the spring beinganchored on an inner part 23 of the body. A hollow tubular drive shaft24 is disposed concentrically within the spring having a radial flange26 which is rotatably held within an annular recess 28 on the forwardpart of the body to allow rotation of the drive shaft but to preventaxial movement thereof. Towards its rear end, the diameter of the shaftincreases at a shoulder 30 in which is provided an anchorage hole 32 forthe rear end of the drive spring. When assembled and prior to first use,the torsion spring 22 is fully energised and acts on the drive shaft torotate it in the dispensing direction. The enlarged portion 34 at therear end of the drive shaft is provided with upstanding splines 36adjacent the shoulder 30, with four of the splines 38 at 90° extendingthe full length of the enlarged portion 34.

At the front end the drive shaft has an internally threaded bore 39 intowhich is threaded the lead screw of a plunger 40. The plunger has anenlarged head 42 which engages the bung 44 of the cartridge 12.Immediately behind the enlarged head 42 the drive shaft has twodiametrically opposed keyways 46 which cooperate with respective keys 48on the front end of the body halves 10 ¹ and 10 ², to allow axialmovement of the plunger, but prevent rotation thereof, as the driveshaft 24 rotates to advance the plunger.

The dose dial 20 is slideably mounted on the enlarged portion 34 of thedrive shaft 24 but prevented from rotation thereof by means of four,internal, equispaced, keyways 50 that engage the longer splines 38 onthe enlarged portion. The forward end of the dose dial has an enlargedinternal diameter so as to be clear of the shorter splines 36. Theexternal surface of the dose dial 20 carries a coarse helical thread 21which threadedly engages a corresponding internal threaded portion 52 onthe inner surface of the dose setting knob 16. The threaded portion 52on the inside of the dose setting knob 16 has a forward limit positionset by termination of the thread (see blind end 54 in FIG. 5). A rearlimit position stop may optionally be provided, for example, on theinner part of a hinged cap portion 17 of the dose setting knob.

The dose setting knob 16 is connected at its forward end to a clutchcollar 56 by means of a snap-fit which allows the clutch and the dosesetting knob to rotate relative to each other but secures them againstrelative axial movement (see FIG. 6). The clutch collar 56 is mountedfor axial sliding movement in the body 10 but prevented from rotationwith respect thereto by means of two lugs 58 in the body engagingopposed respective slots 60 in the cylindrical wall of the clutch collar56. At its rear end, the clutch collar 56 has an internal splinedarrangement 57 designed to slide into splined engagement with thesplines 36 on the enlarged portion 34 of the drive shaft so as to lockthe drive shaft against movement under the influence of the spring byreacting the thrust into the body by via the lugs 58. Referring now moreparticularly to FIGS. 4 to 6, the forward end of the dose setting knob16 has a series of axially extending fingers 62 which are an axialsliding fit with a series of pockets 64 provided in the body.

The axial lengths of the fingers 62 and of the pockets 64 are carefullyselected having regard to the axial spacing between the internal splines57 on the clutch collar 56 and the external splines 36 on the driveshaft 24 such that, on pulling the dose setting knob 16 out axially, thesplines 57 on the clutch collar 56 engage the splines 36 on the driveshaft 24 some distance before the fingers 62 of the dose setting knob 16are withdrawn axially from the pockets 64 on the body. The reason forthis is that, in the position shown in FIG. 2, as indicated above, thethrust of the drive spring is transmitted via the drive shaft splines 38to the dose dial 20 and thence to the dose setting knob 16 (by virtue ofthe dose dial being at its forward limit position relative to the dosesetting knob). It is therefore important to ensure that the clutch 56has engaged the drive shaft 24 to react the load of the torsion spring22 through the clutch 56 and the lugs 58 to the housing before the dosesetting knob has withdrawn enough to rotate.

Once the dose setting knob has been withdrawn far enough to disengagethe fingers 62 from the pockets 64, it is in a setting position in whichit may be rotated to set a dose against a light detent action providedby the fingers 62, which provide a audible/tactile click to enable theuser to count the number of units dialled in. It will be appreciatedthat during dose setting, the body 10, drive shaft 24, clutch 56 allremain stationary, both axially and rotationally. The dose dial is fixedagainst rotation due to its splined engagement with the large portion 34of the drive shaft but is capable of moving axially. Thus in thiscondition, with the dose setting knob 16 axially in its settingposition, rotation of the dose setting knob in the appropriate directionmoves the dose dial 20 axially in a lead screw fashion. The dose dialnumbers are visible through the window 18. The threaded arrangementallows multiple turns of the dose setting knob. Also at this point thereis no loading between the dose dial and the cap and so there is littleresistance to rotary movement of the dose setting knob apart form thatprovided by the detent.

Once the required dose has been dialled in on the helical scale, thedevice is ready for firing. In this arrangement this is achieved bypushing the dose setting knob 16 back in. As the dose setting knob ispushed back in, the fingers 62 re-engage the pockets and thereafter theclutch 56 is shifted forwardly off the enlarged portion of the driveshaft to disengage it so that the drive shaft is now free to rotate. Thedrive shaft rotates by the angular amount dialled in.

In the above arrangement, as the setting of the dose is independent ofthe drive spring, the operating stroke length and force can be reducedto design a device in which a large dose is delivered with a smallmovement and force. This contrasts with existing devices in which theuser has to apply a large user movement to set a dose and/or expel theset dose.

There is a potential issue in existing multi-dose devices in which afterexpulsion of a dose the plunger remains in contact with the bung. Due tothe friction between the bung and the cartridge wall and its resilience,at the end of an injection the bung will be under residual compression.Vibrations transmitted to the bung either via the plunger or the wall ofthe cartridge can cause the bung to expand slightly and expel a dripwhich can affect dose accuracy. As a modification of the aboveembodiment the clutch arrangement can be designed so that, as it ispulled to disengage, it backs off the plunger 40 from the bung 44 afraction, so that the bung 44 is unloaded.

This unloading effect may be achieved for example by putting a slightform on the splines 57 of the clutch collar 56 and on the splines 36 ofthe drive shaft 24 so that, as the dose setting knob is pulled out toengage the clutch collar 56 on the drive shaft, the drive shaft isrotated a few degrees against the spring bias to unload the bung.

Whilst in this embodiment the body is formed in two separate halves itmay instead be formed in a clamshell type arrangement where the twohalves are hinged by a live hinge.

In order to ease manufacture, the torsion spring in the illustratedembodiment may be formed with a wave or compression portion at itsforward end to apply a light axial load to the cartridge in order tohold the cartridge firmly in a forward position. This allows variationsin length due to production variances to be accommodated. Furthermorealthough the wire that is coiled to make the spring may be circular insection in some applications it is preferred for it to be non-circular,such as of square or rectangular section.

Referring now to FIGS. 7 and 8, a second embodiment provides anarrangement for rewinding the plunger 40 and simultaneously energisingthe drive spring 22 (see FIGS. 3 and 4) of the first embodiment. In thesecond embodiment most of the components are similar to those of thefirst embodiment and will not be described again and will be given likereference numerals. Referring to FIG. 7, the forward part of the housingwhich defines the keys 48 that engage the keyways 46 on the lead screwof the plunger is formed as a separate rewind knob 70 which isuni-directionally rotatable about the longitudinal axis of the housing.The knob 70 is formed in two halves 70 ¹, 70 ² and defines a centralaperture with the diametrically opposed keys 48. The knob 70 has anouter circumferential groove 72 in the base of which are twodiametrically opposed sprung ratchet teeth 74, as seen in FIG. 8. Theknob 70 is rotatably received in the forward end of the housing by meansof an inwardly directed toothed annular rib 76 locating in the groove 72and cooperating with the ratchet teeth 74 to allow rotation in onedirection only. The rear end of the rewind knob 70 is provided with twoslots 76 into one of which the front end of the torsion drive spring 22(not shown) is anchored. Engagement of the keys 48 on the knob 70 withthe keyways 46 on the lead screw of plunger 40 ensures that the plunger40 rotates with the knob in one direction only but is capable of axialsliding movement. The knob and the plunger may therefore be rotated inthe direction permitted by the ratchet to wind up the drive spring 22and to retract the plunger back into the driveshaft 24 by virtue of thethreaded engagement between the lead screw of the plunger 40 and thedriveshaft.

The rewind knob will normally be accessible only when the cartridgehousing (see first embodiment) has been removed from the body.

In the above arrangements, a torsion spring provides the motive forcefor expelling the useable contents of the cartridge. It would of coursebe possible to use other suitable drive configurations. For example, asshown in FIG. 9, the torsion spring could be replaced by a compressionspring 80 which acts on the plunger 82 to apply a thrust in thelongitudinal direction. In this case, instead of the drive shaft thereis a drive control shaft 84 that is used to modulate forward movement ofthe plunger 82 under the influence of the compression spring 80.

The rear end of the plunger 82 is formed with a threaded portion whichthreadedly engages an internal bore on the drive control shaft 84 sothat as the plunger 82 is urged forwardly by the compression spring thedrive control shaft rotates. The drive control shaft 84 is formed at itsrear end with an enlarged portion 34 provided with upstanding splines 36similar to those of the drive shaft 24 of the first embodiment, andfulfilling a similar function in conjunction with a clutch collar 56,the dose setting knob 16 and the dose dial 20 of the first embodiment.The construction and operation of these elements will not be describedin detail again. As previously, the drive control shaft 84 isalternately held and released by the clutch collar 56 and the dosesetting knob 16.

The invention claimed is:
 1. An injector apparatus for use with acartridge (12) or syringe to deliver a plurality of doses therefrom, theinjector apparatus comprising: a housing (101, 102) configured toreceive the cartridge or syringe; a plunger (40) configured tocooperate, in use, with the cartridge or syringe to express successivedoses; a dose setting arrangement (16, 20) to select a dose volume; anda drive mechanism (22, 24) releasable to advance said plunger inrespective predetermined increments of magnitude to express saidsuccessive doses, wherein said drive mechanism includes a drive spring(22) for imparting movement directly or indirectly to said plunger (40),and said dose setting arrangement comprises a dose setting element (16)moveable in a dose setting routine to define a magnitude of an incrementof movement of said plunger for a given dose, said drive mechanismfurther including a clutch arrangement operable to selectively engageand disengage the drive spring (22) to/from the dose setting element(16), wherein the dose setting element (16) is configured to be operablymoveable between a first position and a second dose-setting position,the dose setting element (16), in the first position, being engaged tothe drive spring (22) to rotate under the influence of the drive spring(22), and a movement of the dose setting element to the seconddose-setting position moves the clutch arrangement into engagement withthe drive mechanism and disengages the dose setting element (16) fromthe drive spring (22) so that, when the dose setting element (16) is inthe second dose-setting position, the clutch arrangement bears a loadexerted by the drive spring (22) and said dose setting element (16) isfreed from the load of the drive spring (22) such that a dose-settingmovement of the dose setting element (16) in said second dose-settingposition for said dose setting routine is not influenced by the drivespring (22).
 2. The injector apparatus according to claim 1, whereinsaid dose setting element (16) is moveable angularly, and an angularposition of the dose setting element (16) relative to the housingdefines said increment of plunger movement.
 3. The injector apparatusaccording to claim 2, wherein said clutch arrangement includes a clutchelement (56) moveable between a disengaged position and an engagedposition.
 4. The injector apparatus according to claim 3, wherein saidclutch element (56) is arranged within the housing to move axially withsaid dose setting element (16), and axial movement of said dose settingelement from the first position to the second dose-setting positioncauses said clutch element (56) to inhibit forward movement of theplunger.
 5. The injector apparatus according to claim 4, wherein saiddose-setting element (16) and said housing (101, 102) includecomplementary abutments (62, 64) that, in a first mode, engage toprevent relative rotation of said dose setting element (16) relative tosaid housing when said dose setting element (16) is in said firstposition and, in a second mode, disengage to allow relative rotation ofthe dose setting element (16) relative to the housing when said dosesetting element is in said second dose-setting position.
 6. The injectorapparatus according to claim 5, wherein upon moving said dose settingelement (16) from said first position to said second dose-settingposition, said clutch element (56) engages before disengagement of thecomplementary abutments (62, 64) on said dose setting element and saidhousing.
 7. The injector apparatus according to claim 4, whereinreturning said dose setting element (16) from said second dose-settingposition to said first position, after having set a predetermined dosevia a rotation of the dose setting element (16) relative to the housingwhile in said second dose-setting position, disengages the clutchelement (56) and thereby releases the drive mechanism (22, 24) to causesaid plunger (40) to advance by a predetermined increment correspondingto said predetermined dose.
 8. The injector apparatus according to claim3, wherein an operative movement of said clutch element (56) to saiddisengaged position moves said plunger against the load of the drivespring (22) to relieve a residual load between said plunger and saidcartridge or syringe.
 9. The injector apparatus according to claim 1,wherein said plunger (40) is threadedly engaged with an associatedcontrol element (24), whereby linear movement of said plunger isaccompanied by rotation of one of the plunger and the control element,and a magnitude of the increment of movement is controlled by limitingsaid rotation.
 10. The injector apparatus according to claim 9, whereina stop surface (54), associated with said dose setting element (16), isadjustable on movement of said dose setting element to define an angularincrement of movement for relative rotation between the plunger (40) andthe control element (24) upon release of said drive mechanism.
 11. Theinjector apparatus according to claim 10, wherein an intermediateshuttle member (20) is threadedly engaged with said dose setting element(16) and is constrained to rotate with the rotatable one of the plunger(40) and the control element (24), and wherein an extent of relativeangular movement of the shuttle member (20) and the dose setting elementis set by a relative position of the stop surface associated with thedose setting element.
 12. The injector apparatus according to claim 1,wherein said drive spring (22) is a torsion spring.
 13. The injectorapparatus according to claim 1, wherein said drive spring (22) is acompression spring.
 14. An injector apparatus for use with a cartridge(12) or syringe to deliver a plurality of doses therefrom, the injectorapparatus comprising: a housing (101, 102) configured to receive thecartridge or syringe; a plunger (40) configured to cooperate, in use,with the cartridge or syringe to express successive doses; a dosesetting element (16) movable in a dose setting routine to select a dosevolume; and a drive mechanism (22) releasable to advance said plunger inrespective predetermined increments of magnitude to express saidselected dose volume, said drive mechanism (22) including a drive spring(22) for providing motive force directly or indirectly to said plungerto advance said plunger, and said drive mechanism further including aclutch arrangement, the clutch arrangement being movable between a firstposition, in which a clutch element (56) of the clutch arrangement,operable between the housing and a drive shaft of the drive mechanism(22), frees the drive shaft to rotate relative to the housing, and themotive force of the drive spring (22) is transmitted to the dose settingelement (16) and the drive shaft to urge the dose setting element (16)and the drive shaft to rotate, and a second position, corresponding tosaid dose setting routine, in which the clutch element (56) of theclutch arrangement engages the drive shaft to the housing, such that themotive force of the drive spring (24) is transmitted via the clutchelement (56) to the housing (10) and the dose setting element is free torotate independently of the drive shaft and is free of influence by themotive force of the drive spring (22).
 15. The injector apparatusaccording to claim 14, wherein the clutch arrangement is operable, inthe second position, to inhibit forward movement of the plunger.
 16. Theinjector apparatus according to claim 14, wherein a rotation of the dosesetting element (16), when the clutch arrangement is in the secondposition, selects the dose volume expressed upon release of the drivemechanism.
 17. The injector apparatus according to claim 14, wherein thedose setting element (16) is movable between a dose setting position anda rest position, a movement of the dose setting element (16) from therest position to the dose setting position causing the clutcharrangement to move to the second position, the dose setting element(16) being rotatable free of influence from the motive force of thedrive spring (22) when in the dose setting position.